Current density operator only works in non-London GTO basis

Current density operator quantisation currently only works with a SpinOrbitalBasis initialised with `CartesianGTO` (not `LondonCartesianGTO`) basis sets. The reason behind this appears to be a simple hardcoding of the scalar type and GTOShell type expected in `CurrentDensityMatrixElement.hpp` and related functions: ```cpp /** * One of the elements of (one of the vector components of) the second-quantized field-free charge current density operator. * * @param _Scalar The scalar type of the expansion coefficients of the spatial orbitals that underlie this current density matrix element. * @param _Primitive The type of primitive that underlies this current density matrix element. */ template <typename _Scalar, typename _Primitive> class CurrentDensityMatrixElement: public Function<complex, Vector<double, 3>> { public: // The scalar type of the expansion coefficients of the spatial orbitals that underlie this current density matrix element. using Scalar = _Scalar; // The type of primitive that underlies this current density matrix element. using Primitive = _Primitive; // The type of basis function that underlies this current density matrix element. using BasisFunction = EvaluableLinearCombination<double, Primitive>; // The type of spatial orbital that underlies this current density matrix element. using SpatialOrbital = EvaluableLinearCombination<Scalar, BasisFunction>; // The type of the derivative of a primitive. The derivative of a Cartesian GTO is a linear combination of Cartesian GTOs. using PrimitiveDerivative = EvaluableLinearCombination<double, Primitive>; // The type of the derivative of a basis function. using BasisFunctionDerivative = EvaluableLinearCombination<double, PrimitiveDerivative>; // The type of the derivative of a spatial orbital. using SpatialOrbitalDerivative = EvaluableLinearCombination<Scalar, BasisFunctionDerivative>; ``` With `SpatialOrbitalGradient` implemented in #1083, there appears no inherent reason why this functionality should not be extended towards LAOs also.